A shape-memory alloy is an alloy which, when applied with a plastic deformation at a prescribed temperature near the martensitic transformation point and then heated to a prescribed temperature above the temperature at which the alloy reversely transforms into the mother phase thereof, shows a property of recovering the original shape that the alloy has had before application of the plastic deformation. By applying a plastic deformation to a shape-memory alloy at a prescribed temperature, the crystal structure of the alloy transforms from the mother phase thereof into martensite. When the thus plastically deformed alloy is heated thereafter to a prescribed temperature above the temperature at which the alloy reversely transforms into the mother phase thereof, martensite reversely transforms into the original mother phase, thus the alloy shows a shape-memory property. This causes the plastically deformed alloy to recover the original shape thereof that the alloy has had before application of the plastic deformation.
Non-ferrous shape-memory alloys have so far been known as alloys having such a shape-memory property. Among others, nickel-titanium and copper shape-memory alloys have already been practically used. Pipe joints, clothes medical equipment, actuators and the like are manufactured with the use of these non-ferrous shape-memory alloys. Techniques based on application of shape-memory alloys to various uses are now being actively developed.
However, non-ferrous shape-memory alloys, which are expensive, are under economic restrictions. In view of these circumstances, iron-based shape-memory alloys available at a lower cost than non-ferrous ones are being developed. Expansion of the scope of application is thus expected for iron-based shape-memory alloys in place of non-ferrous ones under economic restrictions.
In terms of the crystal structure of martensite into which an iron-based shape-memory alloy transforms from the mother phase thereof by application of a plastic deformation, iron-based shape-memory alloys may be broadly classified into a fct (abbreviation of face-centered-tetragonal), a bct (abbreviation of body-centered-tetragonal), and a hcp (abbreviation of hexagonal-closed pack).
As iron-based shape-memory alloys which transform from the mother phase thereof into a fct martensite by applying a plastic deformation, iron-palladium and iron-platinum alloys are known. These iron-based shape-memory alloys show a satisfactory shape-memory property.
As iron-based shape-memory alloys which transform from the mother phase thereof into a bct martensite (hereinafter referred to as the ".alpha.'-martensite") by applying a plastic deformation, iron-platinum and iron-nickel-cobalt-titanium alloys are known. The .alpha.'-martensite is a phase which is formed in an alloy having a high stacking fault energy, resulting in a large volumic change upon transformation. A slip deformation therefore tends to occur in the .alpha.'-martensite upon transformation, and these iron-based shape-memory alloys do not show a satisfactory shape-memory property in the as-is state. It is however known that, by making the mother phase of these iron-based shape-memory alloys have the invar effect (i.e., a phenomenon in which a thermal expansion coefficient is reduced to the minimum within a certain temperature region), a slip deformation in the .alpha.'-martensite in these alloys is inhibited, and as a result, these alloys can show a satisfactory shape-memory property.
As iron-based shape-memory alloys which transform from the mother phase thereof into a hcp martensite (hereinafter referred to as the ".epsilon.-martensite") by applying a plastic deformation, a high-manganese steel and a SUS 304 austenitic stainless steel specified in JIS (abbreviation of Japanese Industrial Standards) are known. The .epsilon.-martensite is a phase which is formed in an alloy having a low stacking fault energy, resulting in a small volumic change upon transformation. No slip deformation therefore tends to occurs in the .epsilon.-martensite upon transformation, and these iron-based shape-memory alloys show a satisfactory shape-memory property.
As an iron-based shape-memory alloy which transforms from the mother phase thereof into the .epsilon.-martensite by applying a plastic deformation, the following alloy has been proposed:
An iron-based shape-memory alloy, disclosed in Japanese Patent Provisional Publication No. 61-201,761 dated Sept. 6, 1986, which consists essentially of:
Manganese from 20 to 40 wt. %, PA1 silicon from 3.5 to 8.0 wt. %, PA1 chromium up to 10 wt. %, PA1 nickel up to 10 wt. %, PA1 cobalt up to 10 wt. %, PA1 molybdenum: up to 2 wt. %, PA1 carbon up to 1 wt. %, PA1 aluminum up to 1 wt. %, PA1 copper up to 1 wt. %, PA1 chromium from 5.0 to 20.0 wt. %, PA1 silicon from 2.0 to 8.0 wt. %, PA1 manganese : from 0.1 to 14.8 wt. %, PA1 nickel: from 0.1 to 20.0 wt. %, PA1 cobalt: from 0.1 to 30.0 wt. %, PA1 copper: from 0.1 to 3.0 wt. %, PA1 and PA1 nitrogen: from 0.001 to 0.400 wt. %,
at least one element selected from the group consisting of:
and the balance being iron and incidental impurities (hereinafter referred to as the "prior art").
The above-mentioned iron-based shape-memory alloy of the prior art has an excellent shape-memory property. More particularly, the shape-memory property available in the prior art is as follows: A test piece having dimensions of 0.5 mm.times.1.5 mm .times.30 mm was prepared by melting the iron-based shape-memory alloy of the prior art in a high-frequency heating air furnace, then casting the molten alloy into an ingot, then holding the thus cast ingot at a temperature within the range of from 1,050.degree. to 1,250.degree. C. for an hour, and then hot-rolling the thus heated ingot. Subsequently, a plastic deformation was applied to the thus prepared test piece by bending same to an angle of 45.degree. at a room temperature, and the test piece was heated to a prescribed temperature above the austenitic transformation point. Thus a shape recovering rate of the alloy was investigated: the alloy showed a shape recovering rate of from 75 to 90%.
The prior art discloses the addition of at least one element of chromium, nickel, cobalt and molybdenum to the alloy for the purpose of improving a corrosion resistance of the iron-based shape-memory alloy. However, the prior art has the following problems: In the prior art at least one element of chromium, nickel, cobalt and molybdenum is added to improve a corrosion resistance of the alloy as described above. However, particularly because manganese is added in a large quantity as from 20 to 40 wt. % in the prior art, the improvement of corrosion resistance is not necessarily sufficient. Furthermore, the prior art does not give to the alloy a sufficient high-temperature oxidation resistance which is required when heating the alloy for the purpose of recovering the original shape after application of the plastic deformation. The alloy of the prior art, which contains from 20 to 40 wt. % manganese and in addition chromium, tends to form a very brittle intermetallic compound (hereinafter referred to as the ".delta.-phase") because of the presence of chromium. Formation and presence of this .delta.-phase cause serious deterioration of the shape-memory property, the workability and the toughness of the iron-based shape-memory alloy.
In view of the circumstances described above, there is a strong demand for development of an iron-based shape-memory alloy excellent in a shape-memory property, a corrosion resistance and a high-temperature oxidation resistance, but such an iron-based shape-memory alloy has not as yet been proposed.